Derby Talk

Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories

Our pack would like to tighten down on purchased wheels and someone has proposed the following rule be installed:

"Beveling, rounding, tapering, thinning, perforating, weight reduction or any other altering of the weight, size and shape of the wheel is PROHIBITED AND WILL NOT BE TOLERATED. (Removal of mold marks, sharp edges as well as polishing of hubs and bores is not considered altering as long as it does not change the weight, size and shape of the wheel). If the weight, size or shape of the wheel has been altered you can count on the car being disqualified. Car owners will be informed of the violations and given an opportunity to modify the car to meet these rules (i.e. attach new wheels from another kit). If they are unable to make those modifications to the tech team’s satisfaction in a reasonable amount of time, the car may still race but will be given a time of 5 seconds, thereby disqualifying it from the speed races."

Is there a simple way to measure the wheels with a caliper? If they are less than X, they must be replaced. This seems much more objective to me. Also simpler for the inspector, just measure each wheel.

Is there a better way? I know aspects of this have been discussed before. We have all new car kits for the scouts, so is removing mold marks still an issue?

When I was rewriting our Pack / District rules, I wanted to be reasonably sure that a "stock" wheel right out of the box or sleeve wouldn't be inadvertently out of spec (which was reported to have occurred at last year's District race).

So, I bought (or had on hand) 44 US-made wheels from Official Kits or Scout Store wheel sleeves from 2011-12, and purchased three sets of Chinese-made wheels (with 2011-12 date codes) from three separate craft/hobby store (12 wheels total).

The results: Every single wheel measured somewhere between 1.182" & 1.189" in diameter, using a pre-checked digital caliper (I didn't have a dial or runout gauge, so I couldn't check that). The Chinese wheels had marginally greater thickness & weight.

Our District uses a 1.170" value for minimum diameter (legacy value from earlier Council rules). So I'm now reasonably confident that any wheel coming in under that has to have had something done to it much greater than "light sanding".

In the words of Mr Pope, "Better to use wheel features".
The same scouts are going to win every year because they put the time necessary into building the car. They might have their own track, or tracks, in their garage to test their cars. They might study DT. Building cars and testing them might be their year around hobby. Having wheels turned on a lathe will not give them the win if that's all they do. Also, the rules you are talking about with weight of wheels and using a caliper is not going to give you what I suspect you are trying to get.
EXAMPLE: I have 3 sets of Made in USA derby wheels in front of me for my personal use. They were machined by a gentleman on DT. 1 set is 1.172 OD and 2 sets are 1.173 OD. The total weight of 4 wheels of the 1.173 OD is 9.90 grams , heavy enough to comply with the rules that are being discussed. I suspect these wheels are better than anything you can buy on the internet. What would you do if a scout came in with wheels and he actually achieved the same thing by setting up some kind of wood contraption to sand them and got lucky? He's legal by your rules. I think axles bring on another crazy set of rules. Grooved axles are suppose to be the "silver bullet" in some folks mind and many believe you shouldn't even run them with graphite. Anyone can polish axles as good as anyone else if they spend enough time doing it. Will tungsten be illegal? Lead be illegal? Only weight allowed is zinc plate. Read DT, find out how to do it, buy what you need, and get to work. Everyone in the Packs I deal with know DT is available. Those who want to take advantage of it do so. I'd like to think the boys who win these races have enjoyed spending a lot of time with their adult helper.
Good Luck, Gentlemen.

derbyrookie wrote:Our pack would like to tighten down on purchased wheels and someone has proposed the following rule be installed:

Well, the proposed rule does not actually forbid the purchase of wheels improved by a third party. Maybe that aspect should be explicit if that is the intent?

derbyrookie wrote:We have all new car kits for the scouts, so is removing mold marks still an issue?

It shouldn't be an issue anymore.

derbyrookie wrote:Do you feel that 1.170 is generous and would allow someone to lightly sand while prohibiting most purchased lathed wheels?

MaxV specs lathed speed wheels at 1.170" ± 0.001". Winderby sells improved wheels that average 1.176", which would pass the > 1.170" criterion. One vendor offers "2 sizes either Tall 1.180 or around 1.170". Another vendor advertises "there are only a few mold numbers that can be trued to 1.18" and "these [lathed wheels] will pass any rule for light sanding and some of the toughest rules out there." So some vendors already offer specific products to get around specific diameter limits. And because some vendors do not provide diameter information, it may be hard to say where "most" commercially modified wheels will spec.

It's clear that in the race between the rules & the vendors, the vendors will typically keep right up (even without graphite)

However, I would think that there's still merit in creating some sort of readily-measurable threshold, if for no other reason than to not put Cubs using stock wheels at a severe disadvantage from a 'moment of inertia' standpoint.

Our Pack (242) & District (Four Peaks) uses 1.170" diameter & 9.8 grams / 4 wheels because that was a holdover value from earlier years. And the data seems to clearly indicate that any wheel coming in at around 1.170" didn't just drop out of a Grand Prix Pinewood Derby kit box, given that stock wheels are coming in consistently at 1.185" +- 0.003".

So, we now have a potential situation where a Cub Scout checks in a car with 1.171" wheels. Our Four Peaks rules clearly state no "narrowing, lathe cutting, or machining of wheels (inside or outside) shall be allowed". But it would seem that the wheels on the car in question shrank in some mysterious way. We could ask the Cub Scout what he might have done to modify his wheels, and he might reply "sanding" (which is allowed) or "shaving" (which isn't addressed at all, unless it's considered a subset of "machining"). Would we allow him to race with those wheels? It would be difficult to say "no", unless they're obviously narrowed, tapered, "H" tread cut, or some other evidence of reshaping (which is specifically prohibited). And in all cases, if the wheels violate the rules, the Cub would be given the option of trading the wheels for stock ones available in the pit area, or transferring the car to the appropriate category of Open Class.

In hindsight, going to a 1.180" diameter might be better from a "fairness" standpoint, as all stock BSA wheels (US or CN) to my knowledge will fall into that category. But we've 'locked in' the rules for this season - so we'll see what happens.

Now, a question for the experts: The 9.8 gram minimum weight value in the Four Peaks rules is only 5% lower than the typical 10.3 grams for a "out of the box" set of wheels. Will this 5% consistently make that much of a difference in terms of angular momentum & acceleration, or are other factors (axle/bore friction, alignment) more critical?

Maybe we should go with the 1.180. It sounds like we should also explicitly say "No purchased wheels are allowed". The idea is to eliminate the purchased wheel. Axles are another story...for another thread.

Would you not be giving an unfair advantage to the Master Machinist on DT, and all his friends, with a 1.180 rule if they can acquire wheels at 1.189? If you truly are going to measure the diameter of these wheels, I vote for that 1.170 rule. That would be my favorite.

Speedster wrote:Would you not be giving an unfair advantage to the Master Machinist on DT, and all his friends, with a 1.180 rule ... ?

It would depend on whether the final wording was simply a diameter criterion, or also maintains the admonitions regarding specific alterations. As suggested by Richard, a smaller diameter wheel would have a lower mass moment of inertia, which must provide a speed benefit, all other things being equal. So a 1.170" wheel has a potential advantage relative to a 1.180" or stock wheel, but it would almost certainly be outlawed if the language against "altering of the weight, size and shape" proposed within derbyrookie's unit was maintained.

BSA supply and scout shops now stock wheel shavers, so families can easily turn down their own wheels. If the goal is to forbid professionally modified wheels, a diameter specification may not do that alone. But much more than that would probably inhibit the ability of some motivated families to improve their own wheels using a BSA-endorsed tool. The final wording of the rule should reflect the intended goals of the local program.

If you say "no purchased wheels", then that eliminates any official BSA wheels from the Scout Shop or hobby/craft stores (except for the exact four that came in the kit that I presume was given to the Cub). And if the Cub messes up his block & buys a replacement kit, he's now out of spec. I could see that as being a very difficult rule to enforce.

I don't know if I agree with Speedster that a 1.170" is preferable to a 1.180" (even though our District is using 1.170" this year). Yes, you could still machine a wheel & keep it within the 1.180" spec - but the available range of sizes (and inertiae) is much narrower. I could see that an overly-zealous Cub could shave their wheels below 1.180" and be out of compliance, but if the officials allow a swap-out, then the ability to race is not lost.

And getting back to my earlier question: can someone give a knowledgeable estimate of the advantage of that 5% lighter wheel (9.8 vs 10.3 g), given the disproportionate effect of angular inertia? (I know I took dynamics back in college, but I still don't know how I passed - and that's why my seal says "civil", and not something else. )

The point I was trying to make is no matter what rule is made some teams are going to make it work to their advantage without cheating. Think of building a rail rider by going through hundreds of nails until you find bent ones that you can use. I wonder if 99.99% of scouts really care what rules are made. How many families wonder why you're measuring their wheels and what difference it makes? How many scouts come through your pack where the car obviously was not built for speed? I congratulate you on trying to make the experience as fair as you can for everyone. However, some folks can afford to buy 3 scales, tungsten, 32 wheels and on and on and on. Even if you made a rule that you can do absolutely nothing to the wheels some folks would still have an advantage because they would buy a hundred of them and pick the 3 or 4 that they want. In my humble opinion you are changing nothing in the big picture by putting a number on the diameter of the wheel. Sporty is going to beat you anyway.
I think FS answered your question. Doc Jobe and his "2 perfect" cars and all his mathematics would prove the lighter wheels would win the race.
Best of Luck, Gentlemen.

rcmoeur wrote:If you say "no purchased wheels", then that eliminates any official BSA wheels from the Scout Shop or hobby/craft stores (except for the exact four that came in the kit that I presume was given to the Cub). And if the Cub messes up his block & buys a replacement kit, he's now out of spec. I could see that as being a very difficult rule to enforce.

I don't know if I agree with Speedster that a 1.170" is preferable to a 1.180" (even though our District is using 1.170" this year). Yes, you could still machine a wheel & keep it within the 1.180" spec - but the available range of sizes (and inertiae) is much narrower. I could see that an overly-zealous Cub could shave their wheels below 1.180" and be out of compliance, but if the officials allow a swap-out, then the ability to race is not lost.

And getting back to my earlier question: can someone give a knowledgeable estimate of the advantage of that 5% lighter wheel (9.8 vs 10.3 g), given the disproportionate effect of angular inertia? (I know I took dynamics back in college, but I still don't know how I passed - and that's why my seal says "civil", and not something else. )

The reference I've seen most regarding "purchased wheels" is "third party wheels." This encompasses vendors who buy BSA wheels and improve them before resale as well as sending your wheels to a friend to be improved. Yet it allows your Cub to visit a friend and participate in the improvement. I think this is your intent.

Consistency with your district/council rules should be a primary concern. If the district/council goes astray and specifies a minimum diameter, then you are stuck with it. If you can influence the district/council rules to use wheel features as a guide, e.g. "some portion of each tread mark (serration) on the edge of the wheel tread must remain", then every scout can tell by looking while he is working the wheel whether the wheel is of legal diameter or not.

Tread thickness is a major variable in the angular inertia of the wheels. The improvement in that inertia is most marked until the tread is gone. Then advantage from further reductions in diameter are offset by the necessity of the wheel to spin faster to compensate for the reduced circumference. While the effects are small, so are the margins of victory amongst the top race cars, so a clear limit is needed.

The term "light sanding" irritates me becasue it is so subjective. The knowledgeable builder knows to "lightly sand until the minimum allowed tread thickness is attained."

Speedster wrote:However, some folks can afford to buy 3 scales, tungsten, 32 wheels and on and on and on. Even if you made a rule that you can do absolutely nothing to the wheels some folks would still have an advantage because they would buy a hundred of them and pick the 3 or 4 that they want.

Yes, this point is reflected when saying "The final wording of the rule should reflect the intended goals of the local program." Is the goal "to tighten down on purchased wheels" an implicit desire to control the cost of having a competitive car? These are not the same issues; as Stan suggests, the level of Scout involvement is vastly different when the same amount of money is spent on tools instead commercially prepped wheels. If the true goal is "to tighten down on purchased wheels", that might be addressed by something like: "No commercially or professionally modified BSA components are allowed." (Yes, that is difficult to inspect, but racers are sort of on the honor system anyway.) But the proposed rule didn't quite go there.

rcmoeur wrote:can someone give a knowledgeable estimate of the advantage of that 5% lighter wheel (9.8 vs 10.3 g), given the disproportionate effect of angular inertia?

Yes. But as you suggest it is not trivial; it requires a numerical integration of both the mass moment of inertia of the wheel, and the equations of motion of the car.

For wheel MOI, my approach would be to calculate the moment of inertia of a polystyrene annulus that represents the material removed and subtract that MOI value from a published MOI value for 2009 BSA wheels. However, published values vary and it is not always clear what initial (raw) wheel diameter should be assumed.

With wheel MOI values in hand, Jobe's VR II simulator almost certainly answers the question. However, the analysis is dependent on many variables and assumptions that the analyst gets to pick, so there is no single answer. I have access to VR, and could do such an analysis, but honestly it would be very low on my list of holiday priorities. Maybe sometime after New Year's Day?